EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH. Addendum to proposal P242 to the ISOLDE and Neutron Time of Flight Committee

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1 EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH Addendum to proposal P242 to the ISOLDE and Neutron Time of Flight Committee SEARCH FOR NEW CANDIDATES FOR THE NEUTRINO ORIENTED MASS DETERMINATION BY ELECTRON CAPTURE CERN-INTC / INTC-P-242-ADD-1 26/09/2011 D. Beck 1, K. Blaum 2,3, M. Block 1, Ch. Böhm 1,2,3, Ch. Borgmann 1, M. Breitenfeldt 4, S. Eliseev 2, V. Fedoseev 5, S. George 6, F. Herfurth 1, A. Herlert 7, H. J. Kluge 1, M. Kowalska 5, S. Kreim 2, D. Lunney 8, V. Manea 8, S. Naimi 9, D. Neidherr 1, Yu.N. Novikov 10, M. Rosenbusch 11, S. Schwarz 6, L. Schweikhard 11, M. Seliverstov 5, A. Sokolov 1, J. Stanja 12, F. Wienholtz 11, R. N. Wolf 11, K. Zuber 12 1 GSI, Planckstraße 1, Darmstadt, Germany 2 MPI für Kernphysik, Saupfercheckweg 1, Heidelberg, Germany 3 Ph ysikalisches Institut, Ruprecht Karls Universität, 69120, Germa ny 4 Institut voor Kern en Stralingsfysica, B 3001 Leuven, Belgium 5 CERN, Physics Department, 1211 Geneva 23, Switzerland 6 NSCL, Michigan State University, East Lansing, MI , USA 7 FAIR, Planckstraße 1, Darmstadt, Germany 8 CSNSM IN2P3 CNRS, Université de Paris Sud, Orsay, France 9RIKEN Research Facility, Japan 10 PNPI, Gatchina, St. Petersburg, Russia 11 Institut für Physik, Ernst Moritz Arndt Universität, Greifswald, Germany 12 Technische Universität, Dresden, Germany Spokesperson: Sergey Eliseev ( sergey.eliseev@mpi hd.mpg.de) Local contact: Susanne Kreim (susanne.waltraud.kreim@cern.ch) Requested shifts: 5 shifts (1 run) 1

2 Introduction: The determination of the neutrino mass from nuclear electron capture (EC) is an exciting alternative to β decay experiments with 187 Re and tritium [1,2]. Unlike the β decay experiments, in which nuclides with the smallest Q β value are preferred, the determination of the neutrino mass from EC nuclides requires the smallest total energy of the emitted neutrino. To date, the best candidate is 163 Ho with a decay energy of Q EC = 2.56(2) kev [3]. However, there is a variety of other potential candidates with Q EC below 100 kev and with expected very small total energy of the emitted neutrino [4,5]. The choice of the best candidate among them is hampered by imprecise knowledge of their Q EC values. With the proposal P242 and the experiment IS473 we have initiated a search for potential candidates (besides 163 Ho) for a determination of the neutrino mass from EC by a determination of the QEC values of the nuclides of interest [4]. The first two candidates we addressed were 194 Hg and 202 Pb. Status report 2011: The Q EC values of EC in 194 Hg and 202 Pb were proposed [4] to be determined by high precision Penning trap mass measurements of 194 Hg, 194 Au and 202 Pb, 202 Tl with ISOLTRAP. Within the beam time allocated to the experiment IS473 we have successfully completed three quarters of the program by having measured the masses of 194 Hg, 194 Au and 202 Pb with an accuracy of a few kev. An overview of ISOLTRAP beam times that were scheduled and performed in the framework of proposal P242 is given in Table 1. Table 1: ISOLTRAP beam times scheduled for proposal P242. Beam time Dedicated for Target/ion source Remark May Pb, 202 Tl UCx/Hot plasma canceled due to power failure July Au UC x /RILIS successful August Hg UC x /Hot plasma by product of experiment IS461 successful October Pb UC /Hot plasma x successful April Hg UC x /Hot plasma unsuccessful no 194 Hg was seen June Pb 202 Tl UC x /RILIS UC x /Surface ionization Direct measurement of the Q EC value. The Q EC value is measured with insufficient accuracy 2

3 The successful runs in July and August 2008 have enabled us to determine the Q EC value of 194 Hg. The resulting new Q EC value is 29(4) kev and substantially deviates from the AME2003 evaluated Q EC value of 69(14) kev. A thorough consideration of the data available in the literature on mass measurements of 194 Hg and 194 Au has led us to the conclusion that such a strong discrepancy between the AME2003 and our Q EC values originates in inaccurate experimental data for the 194 Au mass taken by AME2003 for the evaluation. The total energy of the neutrino emitted in EC in 194 Hg has thus been calculated to be 15(4) kev. With the new value for the neutrino energy one can presently in principle determine the neutrino mass from EC in 194 Hg with an uncertainty of approximately 20 ev, which would be a tenfold improvement of the present limit. Nevertheless, it would still be much worse than the present limit on the antineutrino mass of 2 ev from the tritium experiments. Thus, it can be concluded that 194 Hg is not a suitable nuclide for the determination of the neutrino mass on the level of a few ev. The new QEC value of 194 Hg and all details associated with the measurement have been published in Physics Letters B 693 (2010) 426. Addendum to proposal: In June 2011 we attempted to perform a direct measurement of the Q EC value of EC in 202 Pb by measuring the cyclotron frequency ratio of 202 Pb/ 202 Tl by alternating between the parent and daughter nuclide in the same run. The RILIS ion source was used to create 202 Pb ions, whereas 202 Tl ions were expected to come out of RILIS due to surface ionization on the hot surfaces in RILIS. Unfortunately, the ionization rate of 202 Tl has turned out to be much lower than expected. This has resulted in the determination of the Q EC value of 51(20) kev with by far insufficient accuracy to draw a definite conclusion on the suitability of 202 Pb for the determination of the neutrino mass. Since the mass of 202 Pb was successfully measured in October 2008 with an uncertainty of approximately 3 kev, it remains to only measure the mass of 202 Tl, which has not been measured in the meantime. Beam time request: Based on the experience acquired during the above mentioned runs, we ask for 5 more shifts of on line beam at ISOLDE to measure the mass of 202 Tl. We would like to perform the experiment in 2012 with the RILIS ion source and UC x target. We expect the 202 Tl yield of 10 5 ions/μc, which is sufficient for our experiment. It is sufficient to determine the Q value of EC in 202 Pb with an uncertainty of approximately 3 kev in order to unambiguously state whether 202 Pb falls into the group of the good candidates. Thus, the mass of 202 Tl has 3

4 to be measured with an uncertainty of 2 kev. The measurement is planned to be performed by alternating between the measurements of the cyclotron frequency of reference 133 Csions and the measurements of the cyclotron frequency of 202 Tl. More than 20 such alternating one hour measurements of the cyclotron frequencies of 202 Tl and 133 Cs are needed to acquire sufficient statistics. This corresponds to about 3 shifts of a pure measurement time. The mass measurements of 194 Hg, 194 Au and 202 Pb have shown that the pure measurement time constitutes at most two thirds of the total beam time. Thus, five shifts and the Ramsey scheme [6 8] for fast measurements appear to be realistic to measure the mass of 202 Tl with an uncertainty of few kev. References: [1] E.W. Otten, C. Weinheimer, Rep. Prog. Phys. 71 (2008) [2] A. Monfardini et al., Nucl. Instr. And Methods in Phys. Res. A 559 (2006) 346 [3] A.H. Wapstra, G. Audi and C. Thibault, Nucl. Phys. A 729 (2003) 129 [4] CERN INTC / INTC P 242 [5] S. Eliseev et al., Phys. Lett. B 693 (2010) 426 [6] M. Kretzschmar, Int. J. Mass. Spectrom. 264, (2007) 122 [7] S. George et al., Int. J. Mass. Spectrom. 264, (2007) 110 [8] S. George et al., Phys. Rev. Lett. 98 (2007)

5 Appendix DESCRIPTION OF THE PROPOSED EXPERIMENTT The experimental setup comprises: ISOLDE and ISOLTRAP Part of the Choose an item. Availability Design and manufacturing ISOLTRAP Existing To be used without any modification 5